1. Correct Answer: D. Less than 20°

Rationale: The magnitude of the Doppler shift depends on the cosine of the angle (È, theta) between the direction of motion (i.e., red blood cells in blood) and the ultrasound beam (Figure 17.4). The velocity measured is equal to the true velocity multiplied by the cosine of the angle away from parallel.

The greater the angle away from parallel, the greater the measured velocity differs from the true velocity (see Table 17.1). To minimize measurement error, the ultrasound beam should be as parallel as possible to the target flow. An angle of less than 20° is recommended because it corresponds to a low level of error, less than 6%.

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2012.

2. Mitchell C, Rahko PS, Blauwet LA, et al. Guidelines for performing a comprehensive transthoracic echocardiographic examination in adults: recommendations from the American Society of Echocardiography. J Am Soc Echocardiogr. 2019;32(1):1-64.

2. Correct Answer: D. Speed of sound in the tissue

The Doppler shift is directly proportional to the reflector speed (flow of red blood cells, V), incident frequency (transducer frequency, f₀), and cosine of the angle (Θ) between the red blood cell flow and ultrasound beam (Equation 17.1). The Doppler shift is inversely proportional to the propagation speed (speed of sound in the medium, C). The equation can be rearranged to solve for blood flow velocity (V)—this is the value calculated by the ultrasound machine. The “2” represents a double Doppler shift: the first shift occurs upon the target (red blood cell). The second shift occurs from the reflected ultrasound wave sent back to the ultrasound transducer from the moving red blood cell.

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2012.

2. Oglat AA, Matjafri MZ, Suardi N, Oqlat MA, Abdelrahman MA, Oqlat AA. A review of medical Doppler ultrasonography of blood flow in general and especially in common carotid artery. J Med Ultrasound. 2018;26(1):3-13.

3. Correct Answer: C. It is based on the principles of PWD

Rationale: Figure 17.1 represents color flow Doppler imaging, also described as 2-D, multigated (multiple scan lines) Doppler. Along each scan line within an image plane, an ultrasound pulse is transmitted, and the reflected waves received along each scan line generate measured Doppler shifts. These shifts (which we interpret as velocities) are coded into colors and superimposed on a 2-D image. Blood flow directed toward the transducer reflects a positive Doppler shift and is color coded in shades of red/orange. Blood flow directed away from the transducer reflects a negative Doppler shift and is color coded in shades of blue.